I have an "Edit user" modal that I'm trying to polish. This is the state of things:

My main concern is that it is optional to enter a new password, and this is not made clear. If the "New password" and "Confirm password" are left empty, then the corresponding user password is left unchanged. Presenting the two inputs fields empty suggests that the a new password should always be entered when editing a user.

What (non-verbal?) cues can I give to the user to explain that entering a new password is optional.

I would recommend a simple approach where the password is not open by default but the user if he desires can edit the password field by using the text link as shown below
Facebook has something similar where the user only edits the password field if he needs to

I cannot pull the password for each user, as this would expose it client-side.
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RandomblueJan 12 '13 at 15:02

I am not asking you to pull the password, just show a masked string. Alternatively you dont even need to show the password string, just keep it like how facebook does
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Mervin JohnsinghJan 12 '13 at 15:03

What would be the length of the masked string? It wouldn't necessarily match the length of the current password.
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RandomblueJan 12 '13 at 15:06

That should be fine. Remember all you are doing is informing the user that the password is there and its masked.
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Mervin JohnsinghJan 12 '13 at 15:08